The present invention relates to separation membranes with the ability to separate p-xylene from mixtures including p-xylene, m-xylene, o-xylene and ethylbenzene.
Para-xylene is a chemical intermediate used in the manufacture of resins, pharmaceuticals, and phthalic acid. It is commonly produced by a reforming reaction, along with ethylbenzene and the other xylene isomers, ortho-xylene and meta-xylene. Para-xylene is typically about thirty mole percent of the product composition.
The recovery of purified p-xylene from this mixture is a difficult and expensive process. Crystallization is the most common industrial method for recovering para-xylene. A limitation of this method is that it is relatively energy and capital intensive. Another method involves simulated-moving-bed (SMB) adsorption followed by crystallization. This method is also relatively expensive, especially since the SMB bed must adsorb a large fraction of the total feed.
Pervaporation, perstraction and gas separation methods have been developed for separating various product mixtures. Mixtures to be separated are contacted as liquids (pervaporation) or vapors (vapor permeation) with one side (the feed side) of a pore-free membrane. Suitable membranes show a high permeation capability (permeability) for at least one component of the mixture, whereas other components will not pass or only pass to a limited extent. The driving force for the transport across the membrane is the gradient of the partial pressure of each permeating component between the feed side and the permeate side of the membrane. At the permeate side one obtains under reduced partial pressure a material stream having a composition different from that of the mixture at the feed side to be separated. The methods of pervaporation and vapor permeation can thus advantageously be used to separate mixtures that are otherwise difficult to separate (such as azeotropic mixtures or components having similar boiling points).
These methods generally employ membranes in the form of sheets, tubes, hollow fibers, and in spiral wound configuration. Hollow fiber, asymmetric membranes are particularly attractive in that the fiber provides high surface area per volume and the asymmetric mode allows for less mass transfer resistance as compared to similar homogeneous materials.
U.S. Pat. No. 4,925,459 discloses a mixed matrix polymer membrane incorporating a zeolite (ZSM-5) and the use of the membrane to separate p-xylene from mixtures of p-xylene and m-xylene. The ratio by weight of polymer to zeolite is greater than 1. A limitation of this approach is that, when the membrane includes a polymer/zeolite ratio this high, the membrane tends to be brittle.
It would be desirable to provide additional devices and processes for isolating para-xylene from a mixture of xylenes and, optionally, ethylbenzene, particularly using membrane technology. The present invention provides such processes.
In its broadest aspect, the present invention is directed to a mixed matrix composite (MMC) membrane capable of separating p-xylene from ethylbenzene and other xylene isomers, and a process for purifying p-xylene using the membrane.
The composite membrane includes a polymer and small, non-interconnected zeolite particles encapsulated in the polymer. The composite membrane is preferably in the form of a sheet, tube or hollow fiber.
The polymer is a polymer that permits passage of p-xylene and m-xylene such that p-xylene diffuses at the same or a faster rate through the polymer. The polymer is generally a polyaramide, polyimide or cellulose polymer.
The zeolite is a zeolite through which p-xylene and m-xylene diffuse, albeit at different rates. The composite permits p-xylene to diffuse through at a faster rate than m-xylene. The zeolite is preferably an intermediate pore size zeolite and, more preferably, is silicalite or ZSM-5. The ratio of zeolite/polymer is typically between about 0.05 and 0.4, preferably about 0.2 to 0.4, by weight. Membranes with significantly higher zeolite/polymer ratios (for example, about 0.65 or higher) tend to be brittle.
A preferred method for preparing polymeric films is by dispersing the zeolite in a polymer solution, casting a film of the polymer solution, and evaporating the solvent to form a polymeric film.
One method for preparing hollow fibers is to melt the polymer, mix in the zeolite particles, and extrude the polymer/zeolite blend through a tubular capillary nozzle with a core fluid used for the purpose of retaining the hollow fiber shape.
Another method involves extruding a polymer spin dope (or spinning solution) formulation including zeolite particles through a spinneret to provide a nascent hollow fiber. The nascent fiber is then contacted with a fluid to coagulate the fiber into a polymer membrane, thus entrapping the zeolite particles in the polymer membrane.
A mixture containing p-xylene and m-xylene can be enriched in p-xylene by a gas-phase, perstraction or pervaporation process through the composite membrane. Pervaporation provides superior selectivity.
Preferred conditions for enriching by perstraction are any combination of feed pressure and temperature where the temperature is just below the boiling point at that pressure. Permeate pressures should be below the feed pressure enough to ensure complete vaporization at the operating temperature. Preferable ranges are temperatures between 30 and 150 degrees Centigrade and feed pressures of 14 psia and 35 psia. Permeate pressures have a preferred range of 0 to 10 inches of vacuum gauge.
In addition to purifying p-xylene and m-xylene, the membranes can be used to separate any molecules that are known to be separated using zeolites, by selecting the appropriate zeolite to incorporate into the polymer membrane.